Today, from the perspective of soldering, let’s talk about what points should be paid attention to when designing PCB.
1. Factors affecting PCB welding quality
From PCB design to the completion of all components welding, to become a high-quality circuit board requires the control of PCB engineers, welding process, welding workers and many other links.
There are mainly the following influencing factors: PCB drawing, quality of circuit board, quality of devices, oxidation degree of device pins, quality of solder paste, printing quality of solder paste, accuracy of programming of mounter, mounting quality of mounter, temperature curve of reflow oven, etc.
The insurmountable link of the welding factory itself is PCB drawing.
Circuit designers seldom weld circuit boards, so they can’t get rich welding experience. However, workers in welding factories don’t know how to draw boards, and they only finish production tasks, and they have no mind and ability to analyze the causes of poor welding.
The talents in these two aspects perform their respective duties, and it is difficult to combine them organically.
Second, the suggestion when drawing PCB
Here are some suggestions to avoid all kinds of bad painting methods that affect welding quality.
1. About the positioning hole
Four holes (the minimum aperture is 2.5mm) should be left at the four corners of the PCB, which are used to position the PCB when printing solder paste. It is required that the center of the X axis or Y axis is on the same axis, as shown in the following figure:
2. About MARK point
Mark points should be marked on PCB for placement of mounter.
Location: On the diagonal corner of the board, it can be a circular or square pad, and it should not be mixed with the pads of other devices. If there are devices on both sides, both sides should be marked.
You can also consider adding MARK points to the imposition.
3. When designing PCB, please pay attention to the following points:
A. the shape of Mark point is as follows, which is symmetrical up and down or symmetrical left and right.
The dimensions of B and A are 2.0mm.
C, within the range of 2.0mm from the outer edge of the Mark point, there should be no shape and color change that may cause erroneous identification. (pads, solder paste)
D. The color of the Mark point should be different from that of the surrounding PCB.
E. To ensure the recognition accuracy, the surface of Mark point is plated with copper or tin to prevent reflection. The shape is only marked by lines, and the light spot cannot be recognized.
As shown in the figure below:
4. About leaving 5mm edge
When drawing PCB, the edge of not less than 3mm should be left in the long side direction for the mounter to transport the circuit board. Within this range, the mounter cannot mount devices. Do not place SMD devices in this range.
As shown in the figure:
For the circuit board with double-sided ornaments, consider the problems of device rubbing and pad rubbing during secondary reflow soldering.
It is recommended that no SMD devices should be placed on the side with fewer chips, within 5mm of the long side from the side. If the area of the circuit board is really limited, you can add process edges on the long side.
5. Do not directly make vias on the pads.
There are holes punched in the pads, and solder paste will flow into the vias in reflow soldering, resulting in lack of solder in the device pads and virtual soldering, as shown in the figure below.
6. Label the polarity of diode and tantalum capacitor.
The polarity labels of diodes and tantalum capacitors conform to the regulations, so as to prevent workers from welding in the wrong direction by experience. As shown in the figure:
7. About screen printing and logo
Please hide the device model. Especially circuit boards with high device density. Otherwise, dazzling will affect finding the welding position. The following figure:
Don’t just mark the model, not the label. As shown in the figure below, the placement machine cannot be programmed.
The font size of silk-screen characters should not be too small, and the positions of characters should be staggered to avoid misreading.
8. The IC pad should be extended
For IC packaged in SOP, PLCC, QFP, etc., it is appropriate that the length of the pad on PCB = the length of IC foot ×1.5, so that when soldering iron is used manually, the chip pins are fused with PCB pad and tin. As shown in the figure:
9. About the width of IC pad
For IC packaged in SOP, PLCC, QFP, etc., pay attention to the width of pads when drawing PCB. The width of pad A on PCB = the width of IC foot (Nom. value in datasheet). It is not recommended to widen it, so as to ensure that the distance B between two pads is wide enough to avoid continuous welding. As shown in the figure:
10. Do not rotate the device at any angle.
As the mounter cannot rotate at any angle, it can only rotate at 90℃, 180℃, 270℃ and 360℃.
As shown in Figure B below, when it is rotated by 1℃, the pins of the device and the pads on the circuit board will be staggered by 1℃, thus affecting the welding quality.
1. Problems needing attention when shorting adjacent pins
The short-circuit method shown in the figure below is not conducive to workers’ identification, and it is not beautiful after welding.
If the drawing is short-circuited as shown in Figure B and Figure C and solder resist is added, the welding effect will be different.
As long as each pin is not connected, the chip will have no short circuit and beautiful appearance.
12. About the middle pad under the chip.
For the chip with belly, it is recommended to reduce the middle pad, so that the distance between it and the surrounding pads is increased, and the chance of short circuit is reduced. The following figure:
13. Two devices with higher thickness should not be closely arranged together.
As shown in the figure below, this layout will cause the mounter to touch the previously attached device when mounting the second device, and the machine will detect the danger, resulting in automatic power failure of the machine.
14. About BGA
Because BGA package is special, its pads are all under the chip, and the soldering effect can’t be seen outside.
For the convenience of repair, it is recommended to make two hole holes with Hole Size：30mil on PCB board, so that the steel mesh can be positioned during repair.
Warm Tip: The size of the positioning hole should not be too large or too small. It is advisable to make the needle not drop or shake after insertion, and it is slightly tight when inserting, otherwise the positioning will not be accurate. The following figure:
15. About PCB color
It is not recommended to make it red. The red circuit board is white under the red light source of the mounter camera, so it can’t be programmed, which is inconvenient for the mounter to weld.
16. About the small devices under the big devices.
Some people like to arrange small devices under the big devices on the same floor. For example, there is a resistor under the nixie tube, as shown in the following figure:
Such typesetting will make it difficult to repair. When repairing, the nixie tube must be disassembled first, which may also cause the nixie tube to be damaged. It is recommended that the resistor under the nixie tube be discharged to the Bottom surface, as shown in the following figure:
17. About the influence of copper-clad connection with bonding pad on tin melting.
Because copper-clad can absorb a lot of heat, it is difficult for solder to fully melt, thus forming virtual solder. As shown in the figure:
In figure A, the device pad is directly connected to the copper clad; Although the 50Pins connector in Figure B is not directly connected to the copper clad, the solder paste in Figure A and Figure B cannot be fully melted because the middle two layers of the four-layer board are copper clad in a large area.
The body of the 50Pins connector in Figure B is a plastic that is not resistant to high temperature. If the temperature is set high, the body of the connector will melt or deform. If the temperature is set low, the copper-clad will absorb a lot of heat, resulting in the solder paste not being fully melted. Therefore, it is recommended that the pad be isolated from large area copper clad. As shown in the figure:
18. Suggestions on jigsaw puzzle and processing edge
summary
Nowadays, there are more and more engineers who can use software to draw, design and wire PCB, and the design is completed, which can improve the welding efficiency. The author thinks that the above factors need to be paid attention to.
Cultivating good drawing habits and being able to communicate well with processing plants is something that every engineer should consider.